This invention relates generally to scanning probe systems, such as scanning probe microscopes and profilometers, and more particularly to the probe assemblies used in these scanning probe systems.
Scanning probe microscopy (SPM; also known as atomic force microscopy (AFM)) is considered a spin-off of scanning tunneling microscopy (STM). An SPM system measures the topography of a sample by scanning (sliding) a probe having a small tip over the sample""s surface and monitoring the tip position in the z-direction at each point along the scan path. Alternatively the SPM probe can be used as a nano-Spreading Resistance Probe (nano-SRP), used for the determination of the resistance and carrier profile of a semiconductor element, or for nano-potentiometry measurements of the electrical potential distribution on a semiconductor element.
FIG. 24 is a perspective view showing a conventional SPM system 40. SPM system 40 includes a movable XY stage 42 for supporting a sample 45, a probe 50 mounted to a suitable structure (holder plate) 60, a probe measurement device 70, and a computer/workstation 80 that serves as both a system controller and a measurement data processor. Holder plate 60 is movable in the z-axis direction by a suitable motor (e.g., a piezoelectric device) to selectively position probe 50 relative to sample 45. Similar motors (not shown) drive XY stage 42 in the xy-plane, thereby causing probe 50 to scan along the upper surface of sample 45, when the probe is in the lowered position. Computer 80 generates control signals that are utilized to control the movements of holder plate 60 and XY stage 42. In most conventional SPM systems, the up-and-down motion of probe 50 is detected by measurement device 70 using the so-called xe2x80x9coptical leverxe2x80x9d method, wherein a laser beam LB generated by a laser 72 shines onto a cantilever surface of probe 50, and the reflected beam hits a two- or four-segment photodiode 75. Measurement data generated by photodiode 75 is passed to computer 80, which processes the measurement data, and typically generates a magnified view of the scanned sample.
FIG. 25 shows probe 50 in additional detail. Probe 50 includes a holder chip (mounting block) 51, a straight cantilever section (stylus) 52 extending from holder chip 51, and an xe2x80x9cout-of-planexe2x80x9d tip 55 that extends perpendicular to cantilever section 52. Probe 50 is supported by holder block 60 at an angle to facilitate contact between tip 55 and an upper surface of sample 45. The choice of the materials from which holder chip 51, cantilever section 52, and tip 55 are composed depends on the type of measurement the probe is intended for. For topography measurement, a dielectric or a semi-conductive tip can be used, whereas for resistance determination and nano-potentiometry require a highly conductive tip, preferably with high hardness and low wear.
One problem associated with conventional probes is that they are expensive and difficult to produce. Conventional probes are typically formed by bulk micromachining high quality, and therefore expensive, monocrystalline silicon (Si) wafers. As indicated in FIG. 25, the relatively large size of each probe 50 is due to the integrated holder chip 51, which is mounted to holder plate 60, and cantilever 52, which must extend from under holder plate 60 to facilitate the xe2x80x9coptical leverxe2x80x9d measurement method. Further, the probes are separated from the Si substrates by etching away the wafer material beneath the probe, which is a time-consuming and costly process. Because of their relatively large size, and because much of the Si substrate is etched or otherwise destroyed during the production process, relatively few probes 50 are formed from each expensive Si wafer, thereby making the cost of each conventional probe 50 relatively high.
Another problem associated with conventional probes is that out-of-plane tips 55 must be fabricated during a separate process from that used to form holder chip 51 and cantilever section 52, and probe 50 must be mounted onto holder plate 60 at an angle relative to an underlying sample 45. Conventional methods needed to form out-of-plane tips, such as tip 55 shown in FIG. 25, add time and expense to the probe manufacturing process. Most conventional out-of-plane probe tips are either etched out of a material (e.g. Si) or they are molded (a pyramidal mold is formed by anisotropic Si etching, the mold is filled up with a material such as a metal or diamond, the mold material is removed). Further, the tip height is limited to only about 15 xcexcm, so probe 50 must be mounted onto holder plate 60 at an angle relative to an underlying sample 45 to facilitate contact between tip 55 and sample 45. To facilitate this angled probe orientation, conventional holder plate 60 is provided with an angled portion 65 to which holder chip 51 is mounted. This mounting process also takes time, and is required for each probe mounted in an SPM system.
At this moment, there is no other SPM technology available which allows the manufacture of scanning probes on wafer scale that can be used to measure structures with high and super-high topography. An important issue in such processing is often to measure the roughness on the bottom of deep structures, and also the top-bottom step height of the structures. Conventional probe 50 cannot do such measurements for two reasons. First, tip 55 is only 5-15 xcexcm high, which determines the deepest structure that can be measured. Second, cantilever 51 is perfectly straight and in-plane with holder chip 52, which means that the probe would bump against the substrate surface if tip 55 enters a structure deeper than the height of tip 55. Step height measurements are commonly done by profilometers that use special probes (i.e., sharpness as small as 10 nm) that can measure large step heights (e.g., 30 to 50 xcexcm). However, these profilometer probes cost up to ten times as much as SPM probes.
What is needed is a probe structure for scanning probe systems that avoids the problems associated with conventional probes that are described above.
The present invention directed to spring probe assemblies for scanning probe systems (e.g., scanning probe microscopes (SPM) and profilometer systems) that are formed using stress-engineered spring material films. Each spring probe includes a fixed end (anchor portion) attached to a transparent (e.g., glass or quartz) substrate, and a cantilever (central) section bending away from the substrate. Curvature of the cantilever section is selectively controlled to form a long free end terminating in a tip that is located in the range of 15 to 500 xcexcm from the substrate. The probe assembly, which includes the substrate and the spring probe, is then mounted in scanning probe system such that the probe tip is scanned over the surface of a sample. A conventional measurement device (e.g., a laser beam and photosensor array) is utilized to detect tip movement while scanning.
Spring probes of the present invention are formed by forming (e.g., sputtering, chemical vapor deposition, or electroplating) a spring material (e.g., metal, silicon, nitride, or oxide) onto a substrate while varying the process parameters (e.g., pressure, temperature, and applied bias) such that a stress-engineered spring material film is formed with an internal stress gradient in the growth direction (i.e., normal to the substrate). The spring material film is then etched to form an elongated island of spring material, and an anchor portion (fixed end) of the spring material island is then masked. The unmasked portion of the spring material island is then xe2x80x9creleasedxe2x80x9d by removing (etching) a sacrificial material located under the unmasked portion. In one embodiment, the sacrificial material removed during the release process is a separate xe2x80x9creleasexe2x80x9d material layer (e.g., Si, SiNx, SiOx, or Ti) that is formed between the substrate surface and the spring material film. In another embodiment, the spring material film is formed directly on the substrate (e.g., glass), and the substrate itself is etched during the release process. The cantilever portion of the released spring probe bends away from the substrate due to the internal stress gradient of the spring material film, while the anchor portion remains secured to the substrate. Controlling, for example, the thickness of the spring material film produces a selected curvature of the cantilever section. To produce curvature variances and straight sections in the cantilever section, stress-reducing layers are deposited on selected sections of the spring material island prior to release. In yet another embodiment, a substrate is coated with resist and patterned to define the probe area. A material stack (including release layer and spring material film) is then deposited over the entire substrate. A lift-off step (e.g., submersion in acetone and applied ultrasonic agitation) is then used to remove the material outside the probe area. The advantage of the lift-off process is that it works with nearly any metal, whereas the etching process allows only for metals that etch well.
The spring probes of the present invention provide several advantages over conventional probes.
First, the spring probes of the present invention facilitate topography measurements that are not possible using conventional probes. In particular, the long, relatively vertical free end of the cantilever section is able to access and measure structures that are deeper and narrower than those measurable by conventional probes, and can scan very close to structures at edges and even on the sidewalls of deep structures. For example, the spring probes are able to measure deep and/or high-aspect-ratio Micro Electrical Mechanical System (MEMS) devices, and perform non-destructive depth profiling of wafers structured by deep reactive ion etching (DRIE), which are not possible using conventional probes. The long, relatively vertical free end the cantilever section also facilitates measurements in liquids and on biological samples, which are also not typically possible using conventional probes.
Another advantage provided by the spring probes of the present invention arises when the spring probes are formed on transparent (e.g., glass) substrates. As mentioned above, conventional probes are bulk micromachined (i.e., separated from an Si substrate), and therefore require a relatively large base portion that is attached to a holder chip in an SPM system. In contrast, because the substrate of each probe assembly is secured to a holder chip, and because the spring probe is fabricated directly on and released from the substrate, the base portion required in conventional probes is not required, thereby allowing the probe assembly of the present invention to be smaller in geometry than conventional probes. Further, because the substrate extends the entire length of the released spring probe, the substrate serves to protect the probe tip during transportation and mounting on holder chip. In contrast, the tips of conventional probes are exposed and often broken during transportation and mounting.
Further, the spring probes of the present invention are significantly less expensive to produce than conventional probes. The spring probes of the present invention can be fabricated using standard lithographic processes on inexpensive glass substrates, as compared to conventional probes that are typically bulk micromachined from high-quality, and therefore expensive, Si substrates. Conventional probes require etching away the entire substrate beneath each probe, which is a time-consuming and costly process, and yields a relatively small number of probes per wafer. In comparison, the spring probes of the present invention are formed using a relatively inexpensive and space efficient lithographic process that yields a significantly larger number of spring probes per wafer. Another advantage is that spring probe fabrication uses a mask aligner with only topside alignment capability, whereas conventional probe fabrication requires special and expensive aligners with backside alignment capability.
Yet another advantage is provided when the tip of each spring probe is formed xe2x80x9cin-planexe2x80x9d (i.e., by shaping the spring material), as opposed to being formed out-of-plane using a conventional tip-forming technique. Conventional out-of-plane tip forming techniques require depositing or etching tip material (e.g., Si, diamond, or carbon-nanotube) on the spring material island after the spring material film is etched. In-plane tips minimize manufacturing costs because they are formed from spring material located at the free end of the cantilever section during the same etching step used to form the spring material island. Further, in-plane tips allow for narrower probe tips, which facilitates inspection of the probed region, in contrast to conventional probes that require wide tips in order to support out-of-plane tips. In other embodiments, a wider structure is formed at the free end of the cantilever section, and an out-of-plane tip is formed using a conventional method.
Yet other advantages are provided by probe assemblies of the present invention due to their curved or bent shape. Conventional probes are straight, and must be mounted on angled surfaces formed on holder plates to achieve the proper orientation necessary to contact a sample surface in an SPM system. In contrast, the fixed ends (anchor portion) of the curved/bent spring probes are oriented parallel to the sample surface, while the free end of the cantilever section can be aligned perpendicular to the sample surface. In addition to the advantages associated with measuring super-high-topography samples and depth gauging on micrometer scale, this parallel orientation produces unique properties and adds completely new functionality to SPM systems and profilometers. For example, the parallel orientation facilitates economically produced multi-probe arrays that include multiple spring probes, arranged both in-line and/or parallel, formed on a holder chip (substrate) using a single lithographic process. Similar multi-probe arrays using conventional probes would require the tedious and expensive process of separately mounting each conventional probe on angled surfaces of a holder plate. Multi-probe arrays formed in accordance with the present invention provide several cost saving and unique functions. For example, when the spring probes are formed using electrically conductive material (e.g., metal), two- and four-tip parallel probe arrays may be utilized to measure electrical properties on a sample surface while viewing the probed area through the transparent substrate.